The present invention relates generally to chill rolls for web offset printing presses, and in particular to a method for controlling a chill roll system using a heat transfer model and press speed to determine the proper coolant temperature and coolant flow rate for a chill roll.
In web offset printing presses, ink is applied to an elongated web of paper or other material as the web is moved lengthwise through the printing press. The freshly printed web is moved through a drier, which elevates the temperature of the web. The web is then moved through a chill roll assembly in order to cool the heated web and to set the ink. A chill roll assembly typically includes a succession of rolls which are cooled by water or other coolant flowing through the interior of the rolls.
To avoid print defects, it is important to properly control both the temperature of the web exiting the chill roll assembly and the temperatures of the chill rolls. If the web exit temperature is not sufficiently low, for example less than 90xc2x0 F., then post-chill marking may occur. On the other hand, cooling the web beyond a desired exit temperature may be an unnecessary waste of energy. Avoiding chill condensate on chill roll surfaces is also important. If a roll is too cool, ink solvent will condense on the roll surface. If a roll is too hot, the web is not cooled sufficiently and an excessive number of rolls is required to cool the web to the desired exit temperature.
Various methods and devices have been used in an attempt to properly control web and/or chill roll temperatures. U.S. Pat. No. 6,202,556 describes using a mixing valve to maintain a desired cooling water temperature. U.S. Pat. No. 5,571,564 describes using a cooled nip roll in contact with a chill roll to improve cooling efficiency and avoid condensate streaking. U.S. Pat. No. 5,275,103 describes using an electrically charged plate electrode for reducing the air gap between the web and the chill roll surface so as to improve cooling efficiency and reduce ink solvent vapors between the web and the chill roll. U.S. Pat. No. 5,465,661 describes adjusting the flow rate of a coolant pump to maintain press temperatures. U.S. Pat. No. 5,918,541 describes warming the coolant upon a web stop to avoid the formation condensation from the air.
DE 197 10 124 A1 describes a chill roll system having separate coolant circuits, whose temperature and flow rate can be independently controlled. DE 88 05 176 describes a chill roll system in which two chill rolls are connected in series with each other and in parallel with two other chill rolls. It is known to use the measured temperature of the coolant to control the chill roll system, as in the HWS HCR-9 chill roll system of Heidelberg Web Systems. Moreover, using Baldwin chill roll wipers to remove condensate from the chill surface is known.
The prior systems and methods tend to be iterative, reacting to temperature changes. These prior systems and methods cannot predict thermal loading at speed changes to determine the required coolant temperatures and flow rates.
The present invention provides a method for controlling a chill roll system in a web printing press. The method includes calculating, based on a web inlet temperature of a web entering a first chill roll of the chill roll system, a dew point temperature of an ink solvent vapor of ink printed on the web is calculated. The inlet coolant temperature of coolant entering the first chill roll is set to temperature value near the calculated dew point temperature. Using a heat transfer model, the flow rate of the coolant through the first chill roll needed to maintain a temperature rise of the coolant through the first chill roll below a predetermined value is determined.
The heat transfer model may be a theoretical and/or an empirical model. The heat transfer model may be adaptive to process measurements.
The first temperature value may be equal to an offset below the calculated dew point temperature.
The method according to the present invention may further include performing the calculating, setting and determining steps for at least a second chill roll of the chill roll system.
Moreover, the method may include calculating a web exit temperature of the web exiting the first chill roll. When the calculated web exit temperature is close to a desired web exit temperature, the method may include setting respective inlet coolant temperatures of coolant entering chill rolls downstream of the first chill roll to a second temperature value based on the desired web exit temperature and setting respective flow rates of coolant through the downstream chill rolls to a minimal flow rate so as to maintain the web at the desired web exit temperature through the downstream chill rolls. The second temperature value may be equal to an offset below the desired web exit temperature.
Setting the inlet coolant temperature of coolant entering the first chill roll may be performed using at least one of a coolant heat exchanger and a coolant mixing valve of the chill roll system.
A computer processing device may be used to calculate the dew point temperature for vapor of the ink solvent and/or to determine the coolant flow rate of the coolant through the first chill roll needed to maintain the temperature rise of the coolant through the first chill roll below the predetermined value.
The method according to the present invention may further include adjusting at least one of the coolant flow rate and the inlet coolant temperature based on a measured web exit temperature of the web exiting the first chill roll. Moreover, the method may include adjusting at least one of a second coolant flow rate and a second inlet coolant temperature of a second chill roll upstream of the first chill roll based on a measured web exit temperature of the web exiting the first chill roll. Still moreover, the method may include adjusting at least one of the coolant flow rate and the inlet coolant temperature based on a measured outlet coolant temperature of coolant exiting the first chill roll.
The method according to the present invention may further include manually adjusting at least one of the coolant flow rate and the inlet coolant temperature based on at least one of a measured temperature of a surface of the first chill roll, a measured temperature rise of the coolant through the first chill roll, and a measured web exit temperature of the web exiting the first chill roll.
The present invention also provides a web printing press having a chill roll control apparatus for a chill roll of a web printing press. The chill roll control apparatus includes a temperature sensor for detecting a web inlet temperature of a web entering the chill roll; and a computer processing device for calculating, based on the web inlet temperature, a dew point temperature for vapor of an ink solvent of ink printed on the web; and determining, using a heat transfer model, a coolant flow rate of a coolant through the chill roll needed to maintain a temperature rise of the coolant through the chill roll below a predetermined value. A controllable heat removal system is also provided for setting an inlet coolant temperature of the coolant entering the chill roll to a first temperature value based on the calculated dew point temperature.
The method and apparatus according to the present invention prevent post-chill marking by sufficiently cooling the web, and sets chill roll temperature profiles to avoid solvent condensation on chill roll surfaces and thereby avoid condensate marking.